new source shapes

Following issue #110 (closed), this is an attempt to improve the particle source by adding new shapes. The user can either choose a predefined shape (point, beam, square, sphere) in the main configuration file or use an external Geant4 macro with /gps/ commands.

To do:

• Add sphere shape
• Add square shape
• Add point shape
• Add use of Geant4 macro
• Check gps commands in macro
• Choose names for the new parameters
• Set alias for old parameter names